As you recall, "Biosphere" refers to the whole of life on Earth; it is commonly described as the envelope or thin sphere around the planet where life occurs. The sphere of life includes the lower atmosphere, the entire world ocean, the entire surface of all lands, and the upper edges of Earth's rock crust.

The Gaia Theory says that the Biosphere as a whole regulates the conditions of life toward the optimum. The Biosphere behaves as if it were a single living system, a super-organism named Gaia (named for Greek goddess Gaia, earth's mother). Some scientists use the term "geophysiology" interchangeably with Gaia.

Giving Earth's homeostatic processes the name "Gaia" was a poor choice. It introduced religion into what became an angry debate, which was less about the science of geophysiology than it was about dogma.

Gaia Theory does not suggest that Earth is conscious or thinks. Feedback loops require no thought or self-awareness. But the theory does argue that the atmosphere, the oceans, the land, and the life in them are closely coupled systems that respond to one another as if they were all part of one body. Walter Cannon, who coined the word homeostasis, called it "the wisdom of the body." Notice again that no thought process is involved in homeostasis. If you stand on one foot for awhile, the balancing act you do works through feedback loops continually correcting the balance, but you certainly don't need to think about it. If you did, you would probably lose your balance.

As Fritjof Capra points out, what is outstanding about Gaia Theory is that it demonstrates that Earth's feedback loops interconnect living systems with non-living systems. In other words, life and the planet have co-evolved.

Nothing on earth is truly separated from the whole.

So what exactly is being regulated by the Biosphere to make the earth hospitable to life?

the amount of salt in ocean water

the amount of carbon dioxide in the air

the temperature of Earth's surface

the amount of oxygen in the air

The Biosphere no doubt regulates many other processes that are not yet recognized.

The major living players in this regulation are microscopic in size. They are bacteria and algal protists.

When we speak of the whole Biosphere as a living system, we must remember that it is enormous and incredibly complex. Feedback loops do regulate toward homeostasis, but there are so many interactions involved that they do not diagram well. However, the carbon cycle diagram, below, suggests that all major processes of Earth take part in her balancing.

One of the puzzles of science for a time came from a discovery about our sun. The sun's radiation had increased by 25% over the past millions of years.But Earth has maintained a constant temperature within 15 degrees F. over this time.The puzzle is this: why hasn't Earth become hotter? What was going on that kept Earth from heating up?

Carbon burial was one big reason Earth did not heat up.

Increased albedo (how reflective or shiny is Earth's surface) was another.
Both reasons result from the process diagramed above. All of these interlocking systems are always active and fluctuating. We could begin anywhere, but begin at Water Cycle, above:

Water is carried from the oceans in clouds, falls on land, weathers rock particles, dissolves them. (Dissolving rock in soil is done partly by bacteria.) Water transports dissolved minerals to the oceans via rivers and groundwater.

water-sculpted walls of dry wash

In the Ocean, algae(phytoplankton) and other protists make tiny shells from carbon in the water and CO2 in the air. Plankton lives, reproduces, and dies, in enormous numbers

diatom

Constant rain of tiny calcium and silica based shells onto ocean floor. Vast sediments of tiny shells slowly turn into limestones and chert over many thousands of years.

foramiferan

These heavy rocks slowly sink (subduct) at the boundaries of tectonic plates.

Limestone subducts into the Mantle under Earth's crust, its buried carbon to emerge some time as carbon dioxide from volcanoes.

Increased carbon dioxide in air stimulates shell algae to pull carbon from both the water and the air, lowering the CO2

Algae release a sulfur compound into the air, which becomes microscopic droplets.

Each droplet helps water vapor condense around it to form raindrops, then clouds. (Raindrops require a nucleus to form around).

Above is one scenario. There are many. Sometimes limestone goes through uplift, which is the tectonic plate process of collision and plate edges rising up to become mountains. After eons, rain and soil bacteria have dissolved the mountains, which wash into the ocean, providing many minerals to ocean life. Carbon is taken up by the shell algae, diatoms, and the process continues.

Earth's homeostasis is the result of extremely slow processes. In human terms, almost forever. But when we knock a system off kilter and cause something like global warming, we want quick fixes. Clever as we are, there may not be any.

The Goldilocks Effect

If conditions on Earth were just a little different, life would not be possible here. You recall that in the story of Goldilocks and the Three Bears, Goldilocks was never happy unless things were just right. Earth’s “just right” conditions for life are called by Earth Scientists the Goldilocks Effect. You could also call it a triumph of balance.

Liquid Water

Earth is just the right distance from the sun for water to be liquid; if the Earth were closer to the Sun, like Venus, the water would be vaporized; if farther, like Mars, it would turn to ice. Living tissue, of course, is mostly water.

Without liquid water, no life. Without liquid water, no geological erosion, transport and deposition on Earth’s surface; no minerals available to build bodies.

Gases Balance

Earth's atmosphere has a mix of gases that maintains a temperature range "just right" for life: the gases maintain a remarkable balance.

Oxygen %

Oxygen, essential to almost all life, is an example. If oxygen were increased by 4 percent, virtually everything on Earth would go up in flames at the first lightning flash.

Lower concentrations of oxygen would slow down or eliminate the chemical processes needed for living things to function.

Carbon Dioxide
%

Carbon dioxide provides another example of fine balance. At less than one-half of 1 percent of atmospheric volume, it is still essential to keeping temperatures warm enough for life through its role in trapping some solar radiation.

When carbon dioxide levels reach just one percent, however, a runaway greenhouse effect can take hold, eventually leading to a climate like that of Venus.

Earth's Mass

Earth’s mass is large enough so that its gravity can hold an atmosphere (gravity holds the air close to the planet). An atmosphere allows the fluid cycling of elements, as in the water cycle. If Earth were much larger, its gravity would hold an atmosphere too dense to admit light from the sun; the surface would be too dark for photosynthesis.

Some thinkers have pointed to the Goldilocks Effect as an argument for the existence of a Creator, claiming that such a fine and unlikely balance could not have happened randomly. Others point to the Goldilocks Effect to argue against the presence of life elsewhere in the Universe. But the Universe is large and marvelous and has room for almost infinite possibilities.